Communication systems utilizing direct transmission of signals from orbiting satellites to Earth-based receivers are well known. Such systems have been described in U.S. Pat. No. 5,121,503 to Davis, entitled "Satellite Signaling System Having a Signal Beam with a Variable Beam Area," and in U.S. Pat. No. 5,239,670 to Schwendeman et al., entitled "Satellite Based Global Paging System." Said U.S. patents are hereby incorporated herein by reference.
A radio signal transmitted from a non-geostationary satellite, e.g., a Low Earth Orbit satellite, can present reception difficulties due to large Doppler frequency shifts caused by orbital motion of the satellite. Because of the high cost of transmitter power in satellite communication systems, earth based receivers used in such systems require high sensitivity to achieve adequate system gain. High sensitivity can be achieved by utilizing a narrow bandwidth, phase coherent receiver, such as a binary phase shift keyed (BPSK) receiver, and further by utilizing differential encoding for the transmitted data.
Unfortunately, the Doppler frequency shift in such a system can be significantly larger than the receiver bandwidth required for high sensitivity. This condition makes it impossible for the receiver to acquire the radio signal except during a brief period in which the satellite is most directly overhead, and thus is producing a limited velocity component with respect to the receiver. Without the Doppler frequency shift, it would be possible for the receiver to receive the satellite transmissions for a much greater period during which the satellite antenna is "in view" of the receiver antenna.
Thus, what is needed is a satellite receiver system that provides acquisition and frequency tracking of a Doppler shifted radio signal to allow narrow-band reception of the radio signal from an orbiting satellite for substantially the entire period during which the satellite is in view of the satellite receiver system.